Enzymatic synthesis and preliminary evaluation as coating of sorbitol-based, hydroxy-functional polyesters with controlled molecular weights

By using a combination of bio-based monomers (sorbitol, 1,10-decanediol and a range of dicarboxylic acids), a series of novel sorbitol-based polyesters was prepared by solvent-free enzymatic polycondensation using an immobilized form of Candida antarctica lipase B (Novozyme 435). The aim was to prepare linear polyesters with pendant, curable hydroxyl groups along the polymer backbone. To achieve this, the polyester molecular weight was controlled by tuning the reaction time, enzyme loading and reaction stoichiometry. Extensive molecular and thermal characterization was performed, showing that the obtained polyesters were semi-crystalline materials with a low Tg. The presence of sorbitol in the polyesters was confirmed through a qualitative investigation using MALDI-ToF-MS. The quantification of the sorbitol content in the polymers was achieved by inverse-gated decoupling 13C NMR spectroscopy, while 31P NMR provided information regarding the selectivity of CALB for the primary vs. the secondary hydroxyl groups. Moreover, 31P NMR and potentiometric titration were utilized for the quantitative determination of the amount of carboxylic groups and hydroxyl functional groups present in the polyesters. The obtained hydroxyl-functional polyesters had suitable properties to be applied as solvent-borne coatings in terms of their molecular weight, functionality and thermal characteristics. Cross-linked coatings were prepared using different conventional curing agents, including two renewable diisocyanates (ethyl ester l-lysine diisocyanate and dimer fatty acid-based diisocyanate). The resulting poly(ester urethane) coatings were tested in terms of solvent resistance, hardness and resistance against rapid deformation, showing the beneficial effect of the implemented sorbitol on network formation